Methods for chemoselective formation of amide bonds between unprotected peptide fragments have provided valuable access to recombinant proteins and have become reliable methods for the synthesis of bioconjugates (Muir, Annu. Rev. Biochem. 72:249-289 (2003); Davis, Science 303:480-482 (2004); Nilsson et al., Annu. Rev. Biophys. Biomol. Struct. 34:91-118 (2005); Pattabiraman et al., Nature 480:471-479 (2011); Stephanopoulos et al., Nat Chem Biol 7:876-84 (2011); Hackenberger et al., Angew. Chem. Int. Ed., 47:10030-74 (2008); Ogunkoya et al., Angew. Chem. Int. Ed. 51:9693-97 (2012); Tam et al., Peptide Science 60:194-205 (2001); Mao et al., J. Am. Chem. Soc. 126:2670-71 (2004)). These advances have made total synthesis of therapeutic peptides and proteins, hormones, and modified antibodies a credible objective (Wang et al., Science 342:1357-60 (2013); Payne et al., Chem. Commun. 46:21-43 (2010); Scheck et al., ACS Chem. Biol. 2:247-51 (2007); Gamblin et al., Chem. Rev. 109:131-63 (2008); Chalker et al., Acc. Chem. Res. 44:730-41 (2011); Kiessling et al., Annu. Rev. Biochem. 79:619-53 (2010); Dawson et al., Annu. Rev. Biochem. 69:923-60 (2000); Raibaut et al., Chem. Soc. Rev. 41:7001-15 (2012)). The key concept underlying peptide ligation approaches is that the amide bond formation step can be accelerated by capturing the carboxyl and the amine functionalities and enforcing intramolecular bond formation (FIG. 1). Various ligation technologies (Bode et al., Angew. Chem. Int. Ed. 45:1248-1252 (2006); Wang & Danishefsky, Am. Chem. Soc. 134:13244-47 (2012); Shen et al., Nature 465:1027-32 (2010); Noda et al., J. Am. Chem. Soc. DOI 10.1021/ja5018442 (2014); Pattabiraman et al., Agnew. Chem. Int. Ed. 51:5114-18 (2012); Aimoto, Peptide Sci. 51:247-265 (1999); Payne et al., Angew. Chem. Int. Ed. 47:4411-15 (2008)), including native chemical ligation (NCL) (Dawson et al., Science 266:776-779 (1994)) and the Staudinger Ligation (Saxon et al., Org. Lett. 2:2141-2143 (2000); Nilsson et al., Org. Lett. 2:1939-1941 (2000)), as well as auxiliary based methods (Kemp, Biopolymers 20:1793-1804 (1981); Hackenberger et al., Angew. Chem. Int. Ed. 47:10030-10074 (2008); Coltart, Tetrahedron 56:3449-3491 (2000)), utilize this concept and are widely used.
Some shortcomings of these and other methods is that they are not completely general. For example, NCL and most auxiliary-based approaches require the presence of an N-terminal cysteine or modified residues that contain a thiol group (Hackenberger et al., Angew. Chem. Int. Ed. 47:10030-10074 (2008)). Moreover, ligation rate and yield can be inefficient with bulky amino acid residues at the ligation junction (Hackeng et al., Proc. Natl. Acad. Sci. USA 96:10068-10073 (1999)).
The present invention is directed to overcoming these and other deficiencies in the art.